I.P. Marozau

Oxygen permeability of perovskite-type Sr0.7Ce0.3MnO3−δ

Abstract The average linear thermal expansion coefficient (TEC) of perovskite-type Sr 0.7 Ce 0.3 MnO 3− δ , calculated from dilatometric data at 300–1100 K in air, is (11.9±0.2)×10 −6 K −1 , compatible with that of stabilized zirconia. The high electronic conductivity, 230–270 S/cm at 773–1273 K, and the moderate thermal expansion suggest possible application of Ce-doped strontium manganite as solid oxide fuel cell (SOFC) cathode material. The oxygen permeation fluxes through dense Sr 0.7 Ce 0.3 MnO 3− δ membranes at 1273–1173 K were found to linearly increase with the oxygen chemical potential gradient in the range of permeate-side oxygen partial pressures from 21 kPa down to 2×10 −9 Pa, indicating a relatively high stability of this phase. However, the oxygen permeability of Sr 0.7 Ce 0.3 MnO 3− δ , limited by both bulk ionic conductivity and surface exchange at the membrane/gas boundaries, is higher than that of La 0.7 Sr 0.3 MnO 3− δ , but still considerably lower if compared to La(Sr)Fe(Co)O 3− δ perovskites.

Fe4+ formation in brownmillerite CaAl0.5Fe0.5O2.5+δ

A Mossbauer spectroscopy study of brownmillerite Ca2FeAlO5+δ, in combination with the structure refinement from X-ray powder diffraction data, showed that hyperstoichiometric oxygen incorporation into the lattice is accompanied with Fe4+ formation in the perovskite-like layers, which are built of oxygen–metal octahedra and preferably occupied by iron cations. This behavior differs from that of another layered ferrite, Sr4Fe6O13+δ, where the point defects formed due to oxidation are primarily accumulated in non-perovskite layers. The incorporation of even minor amounts of extra oxygen in Ca2FeAlO5+δ leads to decreasing water absorption by the lattice, probably due to blocking of oxygen vacancies located in the tetrahedral layers of brownmillerite phase.

Oxygen Permeability and Thermal Expansion of Ferrite-Based Mixed Conducting Ceramics

In order to evaluate promising directions in the development of mixed-conducting membrane materials for oxygen separation and partial oxidation of natural gas, a series of ferritebased ceramics were studied, including La1-xSrxFe1-yGayO3-δ (x = 0.5 0.8; y = 0 0.4), La1-xSrxFe1-yAlyO3-δ (x = 0.7 1.0; y = 0 0.5), La0.3Sr0.7Fe0.7-xAl0.3CrxO3-δ (x = 0.1 0.2), (Sr2Fe3)1-x(SrCo)xOz (x = 0 0.8), CaFe0.5Al0.5O2.5+δ and Ln3-xCaxFe5O12-δ (Ln = Gd, Y; x = 0 0.5). The maximum oxygen permeation is observed for perovskite-type solid solutions with high oxygen deficiency, which exhibit, however, excessive thermal and chemically induced expansion. As for cobaltiteand gallate-based mixed conductors, the increase in ionic transport is accompanied with increasing limiting role of the surface exchange processes. The stability of perovskite-related ferrites in reducing atmospheres, which is comparable to that of LaFeO3-δ and iron oxide, may be moderately increased or decreased by donoror acceptor-type doping, respectively. In addition, the substitution of iron with cations having a more stable oxidation state, such as Ga 3+ , Al 3+ or Cr 3+/4+ , partly prevents the lattice expansion induced by oxygen nonstoichiometry variations, although the solubility of these dopants in the ferrite lattice is limited. Introduction Dense ceramic membranes with mixed oxygen ionic and electronic conductivity are of significant interest for numerous electrochemical applications, including oxygen generators and electrocatalytic reactors for the conversion of natural gas to synthesis gas [1-5]. Synthesis gas (syngas), a mixture of hydrogen and carbon monoxide, is used as a feedstock for commercial Fischer-Tropsh synthesis of hydrocarbons, methanol synthesis, ammonia synthesis, etc. The conventional industrial technologies for syngas generation are based on catalytic steam reforming and/or partial oxidation of methane. Whilst steam reforming is energy intensive due to a highly endothermic reaction, the main cost of partial oxidation is associated with the oxygen plant. Nowadays the cost for synthesis gas production comprises up to 50 % of the total capital investment in gas-to-liquid (GTL) plants [3,4]. Dense ceramic membranes may combine air separation and partial oxidation in one single reactor, and, thus, substantially decrease the investment for the GTL technology [2-4]. The requirements to membrane materials include high oxygen permeability, stability in a wide range of oxygen chemical potential and in the presence of CO2, SOx and steam, compatibility with catalysts and construction materials, low thermal and chemically induced expansion. A special attention is focused on perovskite-related oxides since their transport properties and stability in various atmospheres vary in a wide range. Perovskite-type mixed conductors with highest oxygen Defect and Diffusion Forum Online: 2004-05-20 ISSN: 1662-9507, Vols. 226-228, pp 141-160 doi:10.4028/www.scientific.net/DDF.226-228.141

The effects of switching time and SrTiO3-xNy nanostructures on the operation of Al/SrTiO 3-x N y /Al memristors

The nanostructure and switching properties of the Al/SrTiO3-xNy/Al memristors have been investigated. It was found that the formation of stacking faults defects in the perovskite structure during plasma nitridation facilitates resistivity switching in Al/SrTiO3-xNy/Al memristors. From thermal emission measurements we observed local heating of the Al/SrTiO3-xNy anode interface (285oC) during switching when the interface becomes rectified due to the formation of locally undoped region. A model for resistivity switching and parameters affecting the memristor operation is discussed considering the electromigration theory. The I-V curves are analyzed taking into account the tunnel barrier formations and an inhomogeneous Schottky barrier modification at the anode interface.

Transport Properties and Thermal Expansion of SrCe(Y)O3-δ Ceramics

Oxygen transport properties of perovskite-type SrCe1-xYxO3-δ (x = 0.05–0.10), exhibiting protonic transport in hydrogen-containing reducing atmospheres and mixed oxygen-ionic and ptype electronic conductivity at oxygen partial pressures close to atmospheric, were studied at 973– 1223 K under oxidizing conditions. The oxygen transference numbers of SrCe(Y)O3-δ in air vary in the range 0.37–0.80, decreasing when temperature increases. The oxygen permeability is significantly affected by the hole conduction, which influences both bulk ambipolar conductivity and surface exchange kinetics. The average thermal expansion coefficients of SrCe1-xYxO3-δ ceramics, calculated from dilatometric data in air, are (11.1–11.3)×10-6 K-1 at 373–1373 K.

Electrocatalytic Behavior of Perovskite-Related Cobaltites and Nickelates in Alkaline Media

Dense ceramic anodes of perovskite-type La1-x-ySrxCo1-zAlzO3-δ ( x = 0.45-0.70; y = 0- 0.05; z = 0-0.20) and K2NiF4-type La2Ni1-xMexO4+δ (Me = Co, Cu; x = 0-0.20), synthesized by the glycine-nitrate technique, were assessed for oxygen evolution in alkaline media. The lowest overpotentials are observed for (La0.3Sr0.7)0.97CoO3-δ, which exhibits a significant oxygen deficiency in combination with high conductivity associated with the A-site cation nonstoichiometry compensation mechanism via Co4+ formation. Perovskite-type cobaltite anodes are essentially stable in alkaline solutions, whilst La2NiO4-based electrodes exhibit degradation at the potentials where the oxygen evolution occurs, probably due to the electrochemical oxygen intercalation in the lattice.